1. Technical Field
This invention relates to a lens barrel in which the front and rear lens groups are separated from each other by a lens shutter housing.
2. Prior Art
In a lens barrel of this type, the front lens group frame is linearly movable along the optical axis, and the rear lens group frame is secured to the shutter housing. A drive ring is rotatably mounted on the shutter housing. The drive ring has a thrust cam while the front lens group frame has a engagement part which is engaged by the cam, or vice versa. The thrust cam and the engagement part are maintained engaged with each other by spring means adapted to urge the front lens group frame toward the drive ring (or the shutter housing). Therefore, as the drive ring is turned, the front lens group is moved along the optical axis for focus adjustment with the aid of the thrust cam.
In order to impart high optical performance to the lens barrel thus constructed, the optical axis of the front lens group should coincide with that of the rear lens group with high accuracy. However, providing coincidence between the optical axes of the two lens groups is considerably difficult because the two frames are separated from each other by the shutter housing, and the shutter housing itself is made up of a plurality of components. In order to move the front lens group frame toward or away from the shutter housing, there is an engagement clearance between the front lens group frame and the shutter housing. Therefore, the position of the optical axis of the front lens group is changed in the range of the engagement clearance, which lowers the optical performance of the lens barrel. This difficulty may be eliminated by reducing the engagement clearance. However, this method will increase the machining cost and the assembly cost, and accordingly the manufacturing cost of the lens barrel.